Titanium and titanium alloys are often used in casings for implantable medical devices. The high strength to weight ratio of titanium provides a lightweight, yet structurally strong casing. Similarly, the corrosion resistant nature of titanium allows titanium casings to survive the corrosive fluids within the body. Titanium can undergo a passivation process to further improve the corrosion resistance of the titanium. An added advantage is that titanium is non-toxic and biocompatible reducing the likelihood that the patient will suffer complications from the implantation resulting from the casing itself. While titanium has many material characteristics that are advantageous for implantable medical devices, the material characteristics of titanium also make forming titanium into the appropriate shape difficult.
The high melting point of titanium makes melting or heating titanium for molding or hot forming titanium into the appropriate shapes impractical for high volume manufacturing. Accordingly, titanium casings are typically made in a cold drawing process where a generally planar blank is shaped into the appropriate cup shape by mechanically deforming a titanium blank. In the drawing process, a recessed die is positioned beneath the blank and a punch is pushed against the blank deforming the center portion of the blank into the shape defined by the recess of the die. The punch typically comprises a metal element that is hydraulically pressed with substantial force against the titanium blank. An inherent drawback of the drawing process is that the high pressure metal on metal contact between the titanium blank and the punch typically results in adhesive wear, or galling, of the surface of the drawn titanium piece. In particular, the deformed edge portions of the drawn titanium piece are particularly susceptible to galling. Galling can weaken the titanium pieces, providing an uneven titanium surface and form areas where corrosion can begin. The drawing process can also harden the titanium piece, making the deformed edge portions brittle and at risk for fracturing or tearing during subsequent drawing or punching processes. As a result, drawn titanium pieces are typically heated with an annealer to smooth the surface of the titanium pieces and soften the titanium prior to additional drawing, punching, or other forming of the titanium pieces.
During the annealing process, the titanium pieces must typically be heated to over 1600° F. to induce a material change in the titanium smoothing the galling and softening the titanium. The high annealing temperature for the titanium typically prevents a continuous annealing process as substantial time is required to heat the titanium pieces to the required temperature. Similarly, the high annealing temperature also requires a substantial cooling time in which the titanium pieces are cooled back to a safe handling temperature before being removed from the annealer. Accordingly, a batch annealing process, in which a plurality of drawn titanium pieces are drawn individually before being heated together as a batch, is frequently used to provide some efficiency to the annealing process. However, the batch process is time consuming and labor intensive requiring operators to manually load and unload a plurality of drawn pieces into and out of the annealer creating a substantial bottleneck in the production process.
Although titanium has numerous material characteristics that make it a superior material choice for many applications, the same material characteristics present numerous challenges for drawing and other forming processes. In particular, the substantial inefficiencies of conventional titanium drawing processes create a need for streamlining the process of producing titanium casings.